The instant invention pertains to the stabilization of acid catalyzed thermosetting resins as used in baked enamels or stoving lacquers by use of hindered amine light stabilizers substituted on the hindered N-atom by a hydroxyl group.
Hindered amine light stabilizers are well known to be effective in stabilizing a host of organic substrates including polymers from the deleterious effects of oxygen and light.
Such hindered amine light stabilizers have been used in stabilization of hot-crosslinkable alkyd or acrylic metallic stoving lacquers (U.S. Pat. No. 4,426,472) and in stabilizing acid-catalyzed stoving lacquers based on hot-crosslinkable acrylic polyester or alkyd resins (U.S. Pat. Nos. 4,344,876 and 4,426,471). None of the hindered amine light stabilizers of these patents possess structures having a hydroxyl group substituted directly on the N-atom of the hindered piperidine ring.
In their industrial uses, enamels with high solids content based on crosslinkable acrylic, polyester, urethane or alkyd resins are cured with an additional acid catalyst. Light stabilizers containing a basic nitrogen group are generally less than satisfactory in this application. Formation of a salt between the acid catalyst and the light stabilizer leads to incompatibility or insolubility and precipitation of the salt and to a reduced level of cure and to reduced light protective action and poor resistance to moisture.